I'm sure the ritual of eating together has deep roots in our psyches. It fires off anchors from pre-history that we can't even begin to recognize on a conscious level. The first cave people that lived as an extended family or tribe probably felt the same comfort in the process.
Oh, and when you're teenagers eating with your cousins it can be a lot of fun as well.
With teenagers it's all about fun, food, and sleep, in any order at any time. At any hour one parent can ask the other, "Are they eating again?", and be answered, "Not 'again', STILL."* It's a cause for celebration, especially when you're lucky enough to have family present.
*Actual conversation between my father and mother when I was a teenager.
(Yes, I was once a teenager! It feels like about three years ago...)